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56.2 AMINO ACIDS

These are amino substituted carboxylic acids. Of various amino acids, the α-amino acids or 2-acids are quite important because they are the building blocks of peptides and proteins. In α-amino acids, the amino group is present on the α-carbon atom (i.e., C atom next to COOH group): Thus, they can be represented by the general formula,

Specific Objectives

After studying this unit, the students will be able to:

  • describe the sources preparation and properties of proteins
  • describe the nomenclature of amino acids.
  • describe proteins as a natural polymer.
  • describe the uses of proteins.

R – CH – COOH

         |

       NH2

Group R – is different for different α-amino acids.

About 20 of the a-amino acids have been identified as the constituents of most of the animal and plant proteins. The formulae of such amino acids along with their common and abbreviated names have been given in Table 56.1.

CLASSIFICATION OF AMINO ACIDS

The amino acids containing equal number of amino and carboxyl groups are known as neutral amino acids. The amino acids which contain more number of carboxyl groups than amino groups are known as acidic amino acids while those containing more number of amino groups are called basic amino acids. For example, glutamic acid and aspartic acid are acidic amino acids while lysine is basic amino acid. Alanine is a neutral amino acid.

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ESSENTIAL AND NON-ESSENTIAL AMINO ACIDS

Out of 20 amino acids which are required for protein synthesis, the human body can synthesize 10. The amino acids which the body can synthesize are called non-essential amino acids. The remaining 10 amino acids which the body is not able to synthesize are called essential amino acids. The essential amino acids must be supplied to our bodies through our diet because they are required for proper health and growth. The deficiency of essential amino acids may cause diseases like Kwashiorkor in which water balance of the body is disturbed.

STRUCTURE OF AMINO ACIDS

It has been found by spectroscopic means that -COOH and –NH2 groups of amino acids do not exist as such but they react with each other to form internal salt structure which is also called zwitter ion structure. In the formation of zwitter ion, a proton from -COOH part of the molecule is released and attaches itself to –NH2 part to constitute a dipolar ion as shown below:

Zwitter ion is a neutral species but carries both positive and negative charges.

 PROPERTIES OF AMINO ACIDS

The amino acids are usually colourless, crystalline and high melting solids. They are moderately soluble in water. They are insoluble in organic solvents such as ether, benzene
and petroleum ether. In acidic solution amino acids exist as cations and migrate towards cathode in an electric field whereas ill basic solutions they exist as anions and migrate towards anode. At the intermediate pH, however, they exist as zwitter ion (a dipolar ion) and do not migrate towards either electrode. This pH is known as the isoelectric point of the α-amino acid.

Different amino acids have different isoelectric points.

The isoelectric point of an amino acids depends upon the functional groups present in the amino acid. Neutral amino acids have isoelectric point in the pH range of 5.6-6.3.

At isoelectric point the amino acids have least solubility in water. This property of amino acids is exploited in the separation of different α-amino acids obtained by hydrolysis of proteins.

α-Amino acids exhibit chemical reactions characteristic of primary -NH2 group and -COOH group.

NAMING THE AMINO ACIDS

Amino acids are known by their common names. However, they can also be named by following the IUPAC rules. In protein or peptides, the amino acid units are represented by a three letter symbol or a one letter symbol. For example, the abbreviation for glycine and alanine are gly, ala respectively. The names, structures, and abbreviations of various α-amino acids are given in Table 56.1.

Table 56.1. Structure and Names of Some a-Amino Acid

H2N – CH – COOH

             |

             R

 

 6 Serine

Ser

S

-CH2-OH
 7 TyrosineAcidic amino acids

Tyr

Y

 8 Aspartic acid

Asp

D

-CH2-COOH
 9 Glutamic acidBasic amino acids

Glu

E

-CH2-CH2-COOH
 10 *Lysine

Lys

K

-CH2-(CH2)3-NH2

 

Activity 56.1

Study of α-Amino Acids

Draw structures of the following a-amino acids

      i.            Alanine

      ii.            Phenylalanine

iii.            Tyrosine

iv.            Aspartic acid

v.            Lysine

vi.            Glutamic acid

Give IUPAC names of these α-amino acids. Use the information from Table 56.1. .

 SOLVED EXAMPLE

Example 56.1 Give reasons for the following:

On electrolysis in acidic solution amino acids migrate towards in alkaline solution these migrate towards anode.

R – CH – COO-

         |

      NH2

R – CH – COOH

         |

     +NH3

Solution. In acidic medium, amino acids exist as cationsand in basic medium they exist as anions

Therefore, on electrolysis in acidic medium they migrate towards cathode while in alkaline medium they migrate towards anode.